Protection relay system against single-phase faults for medium-voltage distribution networks

Abstract

The electrical power distribution network comprises a medium-voltage feeder having an upstream end intended to be connected to a power source. The feeder includes at least first and second consecutive protection relays positioned along the feeder and defining the ends of a first element to be monitored. The first protection relay includes a search module configured to trip a breaker to interrupt the electrical power distribution: after a first time delay when the search module detects a single-phase permanent fault in a monitoring area of the first element; and after a second time delay longer than the first time delay when the search module detects a single-phase permanent fault in an additional monitoring area located downstream of the area.

1 . Electrical power distribution network comprising:
 a medium-voltage feeder having an upstream end and a downstream end, the upstream end being intended to be connected to a power source;   at least first and second consecutive protection relays positioned along said feeder and arranged so that the first protection relay is located between the upstream end of said feeder and the second protection relay, the first and the second protection relays defining the upstream and downstream ends of a first element associated with the first protection relay;   
       wherein the first protection relay comprises:
 a search module configured to detect a single-phase permanent fault downstream of the first protection relay, the search module being provided with a phase current and phase voltage measurement circuit; 
 a computer configured for computing a complex quantity Z i  having a real part Re(Z i ) and an imaginary part Im(Z i ), from the measured phase voltage and phase current, said complex quantity Z i  taking into account the position of the location of the single-phase permanent fault with respect to said measurement circuit, and a resistance value of the fault; 
 a comparator configured for comparing the complex quantity Z i  with first and second complex thresholds corresponding to first and second straight lines in a complex plane associated with reference frame (O, Re(Z i ), Im(Z i )) wherein:
 the first complex threshold defining in the complex plane a first domain configured to represent occurrence of a single-phase permanent fault in a monitoring area comprised in the first element and having the first protection relay as an upstream end, and 
 the second threshold defining with the first threshold in the complex plane a second domain which does not overlap with the first domain, and which is configured to represent occurrence of a single-phase permanent fault in an additional monitoring area distinct from said monitoring area and arranged downstream thereof so that the downstream end of the monitoring area corresponds to the upstream end of the additional monitoring area; 
 
 a breaker for cutting out the electrical power distribution downstream of the first protection relay after:
 a first time delay when the complex quantity Z i  belongs to the first domain of the complex plane; 
 a second time delay longer than the first time delay when the calculated complex quantity Z i  belongs to the second domain of the complex plane, and when the single-phase fault is still detected after the first time delay. 
 
 
     
     
         2 . Electrical power distribution network according to  claim 1 , comprising a third protection relay arranged between the second protection relay and the downstream end of the feeder, the second and third protection relays defining the upstream and downstream ends of a second element associated with the second protection relay, the additional monitoring area is continuous and comprises a portion of the second element. 
     
     
         3 . Electrical power distribution network according to  claim 1  wherein the calculated complex quantity Z is provided by relation: 
       
         
           
             
               
                 
                   
                     
                       Z 
                       i 
                     
                     = 
                     
                       
                         
                           
                             V 
                             
                               i 
                                
                               
                                   
                               
                                
                               Φ 
                             
                           
                           
                             
                               I 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 Φ 
                               
                             
                             + 
                             
                               
                                 k 
                                 i 
                               
                               · 
                               
                                 I 
                                 iR 
                               
                             
                           
                         
                          
                         
                             
                         
                          
                         with 
                          
                         
                             
                         
                          
                         
                           I 
                           iR 
                         
                       
                       = 
                       
                         
                           I 
                           iA 
                         
                         + 
                         
                           I 
                           iB 
                         
                         + 
                         
                           I 
                           iC 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         Φ designating the faulted phase, A, B, or C, of the three-phase system; 
         i designating an index relative to the first protection relay; 
         V iΦ  and I iΦ  representing the voltage and current of the faulted phase, measured by the measurement system of the first protection relay in the presence of the single-phase fault; 
         I iR  representing the residual current which is equal to the sum of the three phase currents measured by the measurement circuit of the first protection relay and; 
         k i  representing a calculation coefficient associated with the first protection relay. 
       
     
     
         4 . Method for protecting an electrical power distribution network according to  claim 1 , comprising the following steps on detection of a single-phase permanent fault by the first protection relay:
 a calculation of the complex quantity Z i  from a phase current and phase voltage measured by the measurement circuit;   a comparison of the calculated value Z i  with the first and second complex thresholds corresponding to straight lines in the complex reference frame (O, Re(Z i ), Im(Z i ));   a verification of the presence of a single-phase permanent fault either in the monitoring area, or in the additional monitoring area.   
     
     
         5 . Method for adjusting the first protection relay of the power distribution network according to  claim 3 , comprising the steps of:
 carrying out a plurality of electromagnetic transient simulations of the feeder on occurrence of a single-phase permanent fault to calculate the phase voltage and phase current in the first protection relay, the simulations being performed by varying at least the location of occurrence of the single-phase permanent fault and the resistance thereof;   calculating the complex quantity Z i  for each simulation;   representing the different calculated complex quantities representative of the simulations in the complex reference frame (O, Re(Z i ), Im(Z i ));   modulating calculation coefficient k i  to vary the different values of the calculated quantity Z i  in the complex reference frame (O, Re(Z i ), Im(Z i )), and to achieve a definition of the first and second straight lines, corresponding to the first and second threshold, delimiting the maximum number of values of the calculated quantity Z i  corresponding to said first and second domains.   
     
     
         6 . Method for adjusting a protection relay of a power distribution network according to  claim 5 , wherein the modulation of calculation coefficient k i  is performed so that:
 for each iteration μ, μ being an integer ranging between 1 and μ max  a maximum number of iterations determined beforehand, evaluation calculations exploiting the results of the electromagnetic transient simulations of single-phase permanent faults in the feeder are carried out for each calculation coefficient k i   μ,λ  belonging to a list Γ μ  of ν coefficients Γ μ ={k i   μ,λ , λ=1, . . . , ν} for each evaluated calculation coefficient k i   μ,λ , first S i1   μ,λ  and second S i2   μ,λ  thresholds associated with the first protection relay are determined and a probability α i   μ,λ  of successful discrimination of the monitoring areas comprising the simulated fault is calculated;   after each iteration μ, a coefficient k i,opt   μ  is selected from list Γ μ , the selected coefficient k i,opt   μ  having the highest discrimination probability α i   μ ;   for each iteration μ, with μ varying between 2 and μ max , list Γ μ  of the ν values of coefficients k i   μ,λ  to be evaluated depends on the selected calculation coefficient k i,opt   μ−1  obtained after iteration μ−1;   the optimization is stopped when the number of iterations reaches μ max  or when the highest discrimination probability α i   μ , determined during iteration μ, is greater than or equal to a probability threshold α opt  determined beforehand;   the modulation calculation coefficient k i,opt  corresponds to coefficient k i,opt   μ  obtained during the last iteration to have been performed.